As materials of devices for boilers, chemical plants and the like, which are used in a high temperature environment, austenitic stainless steels such as SUS 304H, SUS 316H, SUS 321H, SUS 347H and SUS310S, which are standardized in JIS, have been conventionally used. However, in recent years the use conditions of these devices, under such a high temperature environment, have become remarkably severe. Accordingly, the required properties for this material have attained a higher level, and the conventional austenitic stainless steels are markedly insufficient in high temperature strength.
Carbides are useful for improving high temperature strength, particularly creep strength of the austenitic stainless steel, and the strength enhancing effect of carbides, such as M23C6, TiC and NbC is practically used. Further improvement of creep strength by the addition of Cu is also applied because the fine Cu-phase, which precipitates during creeping, can contribute to the enhancing of creep strength.
On the other hand, it has been known that P, which is considered as an impurity of the steel, contributes to the improvement of creep strength due to the refining of the M23C6 carbide. For example, Patent Document 1 discloses an invention wherein P is added for enhancing creep strength. However, since increase of the P content deteriorates weldability and creep ductility, the content of P should be restricted. Therefore, it cannot be said that the enhancing effect of the addition of P is fully used.
Patent Document 1: JP Kokai Sho 62-243742
An austenitic stainless steel containing P of more than 0.06%, but not more than 0.20%, is disclosed in Patent Document 2. The steel has been developed for improving the resistance to salt damage under a high temperature environment. Accordingly, it contains an excessive amount of Si, more than 2.0% but not more than 4.0%. Such a large amount of Si promotes precipitation of the σ-phase, and deteriorates the toughness and ductility of the steel.
Patent Document 2: JP Kokai Hei 7-118810